With the development of better and better barrier materials, generally composites that include a plastic film base, it has now become very desirable to be able to precisely measure the rate of permeation through such barrier materials in order to properly evaluate them. As barrier materials have improved in their resistance to moisture and oxygen permeation, it has become clear that better, more sophisticated methods and apparatus will be required to be able to accurately measure such lower and lower rates of permeation that are expected to be representative of barrier materials that are felt to be needed for commercial applications.
Gas permeability measuring devices have generally been known in this art, some of which were developed to serve the garment industry where the production of fabrics that were highly resistant to water permeation were being developed. However, more recently, with the development of LCD's, LED's and OLED's, it has become important to develop barrier materials that have an extremely high resistance to moisture permeation and oxygen permeation; it has been scientifically shown that there is a relationship between the permeation of moisture and the permeation of oxygen through a barrier so that, by measuring moisture permeation rate, a reasonable assessment can also be obtained for the resistance of the barrier film to the permeation of oxygen.
Products in various electronics fields, such as OLED's and LCD's, and certain pharmaceuticals are among the products for which it is presently felt to be particularly important to minimize exposure to oxygen and moisture in order to resist deterioration of such products. Barrier materials that have been developed to protect such materials generally include multilayer composites made of polymeric films and thin layer inorganic materials, and the search goes on for providing increasingly better multilayer, thin film barrier materials for this purpose. These materials will generally include a thin polymeric film, e.g. PET, that will carry at least one overall inorganic layer. For example U.S. Pat. No. 6,413,645 entitled “Ultrabarrier Substrates” describes the problem and the search for more permeation-resistant materials. However, this patent states that oxygen and water vapor transmission rates even as high as 0.005 cc or gm/m2/day are below the detection limit of current industrial instrumentation. U.S. application No. 2004/0209126 discloses a highly effective barrier film wherein a PET film having barrier coatings of ITO and/or SiO2 is deposited thereupon. By using a specific, ion-assisted, sputtering or evaporation process, the result is an improved structure that exceeds the performance of comparable flexible films.
To measure moisture permeation, U.S. Pat. No. 3,580,067, at an early date used the amount of change in weight of a suitable desiccant in a closed container. U.S. Pat. No. 4,663,969 later disclosed apparatus for testing water vapor transmission which employed a heated water bath and measured the change of solute indicative of moisture permeation by measuring a change in electrical conductivity.
Our U.S. Pat. No. 6,804,989 (Oct. 19, 2004) discloses an apparatus for measuring ultralow water permeation through a composite barrier film that includes a thin polymer layer by utilizing a radioactive gas, such as tritiated water vapor (HTO) or carbon14monoxide (14CO). The sample is mounted to provide controlled access to opposite surfaces of the barrier film, and HTO or 14CO is supplied to its upstream surface. The permeating radioactive gas is collected in a carefully controlled, dry, carrier gas stream and monitored in a manner to precisely determine even extremely low permeation rates through the sample. The method affords highly accurate measurement of ultralow permeation rates by uniformly controlling the humidity (or CO concentration) at the upstream surface and by using a controlled, very low flow of dry carrier gas, preferably having a matching molecular weight, to collect all the radioactive, permeated gas and carry it to a radiation monitor in an ionization chamber where the permeation rate is then calculated.
Although the last described apparatus is able to effectively evaluate the performance of the highly effective barriers involved, there is an inherent need to provide test data for resistance to moisture permeation over long time periods. Although continuous testing for such long periods of time can supply such data, it is undesirable to have to wait until the end of perhaps a year or even 1 month of such testing. The present desire to shorten the time needed to determine a material's extended resistance to moisture permeation has caused the industry to turn to testing for moisture permeation at elevated temperatures. However, testing for such short times at high temperatures has really been used as only a pass/fail test as no definitive data has been generated as a part of such testing. Thus, more accurate apparatus and methods continue to be sought to provide accelerated testing that can provide a reasonably accurate indication of a material's long term barrier properties at ambient temperature in the form of definitive values.